A microscope slide Cavity is a thin flat piece of glass,typically 75 by 26 mm (3 by 1 inches) and about 1 mm thick,used to hold objects for examination under a microscope.Typically the object is placed or secured on the slide,and then both are inserted together in the microscope for viewing.This arrangement allows several slide-mounted objects to be quickly inserted and removed from the microscope,labeled,transported, and stored in appropriate slide cases or folders.